1. Field of the Invention
The present invention resides in the field of rapid prototyping technology, in which three-dimensional objects are formed in various ways responsive to user-defined geometric parameters.
2. Description of the Relevant Art
Various processes have been developed to produce three-dimensional objects, also called models and prototypes, in which an object is formed by a computer-controlled machine in accordance with dimensional parameters defined by the user. Several of these methods, including stereolithography, are described in the background section of U.S. Pat. No. 5,590,454 to Richardson.
In processes such as stereolithography, a liquid polymer is selectively solidified and cured, by means such as a laser, in successive layers until a final object is formed. Once the object has been formed, it is rinsed and then undergoes post-curing, since a laser is not capable of completely curing the material. For instance, U.S. Pat. No. 4,575,330 to Hull teaches use of an ultraviolet ("UV") floodlight to post-cure the formed object. Such post-curing is subject to certain drawbacks. It has been found that UV light cannot fully cure certain resins, such as, for example, epoxy resins cured with a cationic initiator. Consequently, post-curing of the object is necessary to fully cure the material. If such post-curing is done thermally, i.e., by placing the object in a high-temperature setting (such as an oven or autoclave), toxic fumes are produced, which are harmful to the process operator and to the environment generally. Given such drawbacks, an improved means of post-curing is desirable.
The aforementioned patent to Hull suggests possible substitution of an electron source for the UV light source described with regard to the preferred embodiment. In the example of such use, Hull teaches the application of an electron beam to material which has been prepolymerized with UV light. However, there is no disclosure regarding application of an electron beam in a post-curing step. Furthermore, Hull does not mention what dosage rates and energy magnitude would be appropriate to cure material using an electron beam. It has been found that if a low-energy electron beam is used, the material is only partially cured during the object-forming phase, so post-curing of the object would still be necessary. If, on the other hand, a high-energy electron beam is used, the beam would penetrate the liquid polymer to a significant degree, such that a cured discrete layer is considerably thicker than the layers successively obtained with a laser or low-energy electron beam. Such a result diminishes the dimensional precision inherent with the standard or low-energy electron beam stereolithographic process. Finally, a stereolithographic machine employing an electron beam source is not commercially available. Therefore, there is a need in the three-dimensional prototyping field to derive the benefits of electron beam curing with a method which overcomes the foregoing disadvantages.